Various linear polyesters and copolyesters have been prepared for subsequent conversion into products such as films, tapes and sheeting. These polyesters and copolyesters are all polymeric compounds containing in their structural units, ester groups of the type derived from the interreaction of organic dicarboxylic acids or lower alkyl esters thereof and dihydric glycols. Film, tape and sheeting products prepared from these polyesters and copolyesters have very smooth surfaces, a highly desirable characteristic, but because of this smoothness have such an affinity for each other that when wound, stacked or otherwise brought into contact with each other will tend to stick together even when subjected to only the slightest of pressures. This tendency to stick or "block" gives rise not only to problems during the preparation of such products, but also contributes to a diminution in their appearance qualities as a result of surface-to-surface abrasion during subsequent handling of such products.
In order to minimize or prevent the tendency of such products to stick or block together when brought into contact with each other, past practice has been to add to the polyester or copolyester what is commonly referred to in the industry as a slip or nonblocking agent. Generally, slip or nonblocking agents consist of finely dispersed materials which reduce the coefficient of friction between adjacent surfaces of the products when brought into contact with each other. Examples of slip or nonblocking agents which have been employed in the past are such materials as silica, quartz, aluminum silicate, kaolin, calcium phosphates and the like. However, the use of such inorganic materials have two major drawbacks. The first of these is that, being inorganic, these materials are not soluble in the polyester or copolyester and thus impart an objectional haze in the final extruded or molded product. The second drawback is that such inorganic materials are difficult to disperse uniformly throughout the polyester or copolyester, frequently leading to the formation of agglomerates. These agglomerates not only cause difficulties during the processing of the polyester or copolyester into films, tapes, sheeting or other like products but also impart objectionable characteristics to the product such as impairment of clarity, interference with x-ray transmission or interference with audio and visual reproduction. Organic slip agents such as fatty acid amides have also been suggested, but these function by migration of the amide to the surface of the film or tape thereby limiting the uses to which polyesters containing these agents can be applied.
In contrast to past practice the present invention offers an attractive alternative to the use of inorganic and/or organic slip agents and the drawbacks associated therewith.
The inventor has found that when a block copolyester as taught in U.S. Pat. No. 4,348,510 is extruded into a film and subsequently biaxially oriented, the film suffers from commercially unacceptable static coefficient of friction, however, it has acceptable levels of haze. The inventor has also found that in levels in excess of 12% by weight that the block copolyester when extruded and subsequently biaxially oriented, may be unacceptable due to a high percentage of haze (excess of 14%) while having excellent static coefficient of friction. Unlike the copolyesters disclosed in U.S. Pat. No. 4,348,510, the copolyesters of the present invention do not have to be "conditioned" in accordance with ASTM test method D-1894-78 in order to exhibit improved nonsticking properties. Accordingly, the inventor has unexpectedly found that levels from 5.0% to 12% in a copolyester which is then extruded into a film and subsequently biaxially oriented has excellent slip characteristics in the absence of conditioning as well as maintaining excellent haze values (from 3.5% to 14.0%).